The difficulty of mastering the Pathfinder technique could not be overstated. While the data processing needed to use the Pathfinder was still not as extreme as that of the VOID algorithm, it was far from something a Martial Squire could accomplish.
In fact, if it wasn't for the fact that Rui had mastered the technique, it could probably be considered a Senior-level technique!
After all, by definition, Senior-level techniques were techniques that only Martial Seniors and above could master. If not for Rui, it would most likely be the case that Martial Squires simply didn't have what it took to master the technique.